The present invention refers to the field of controls for electrical machines and in particular refers to a control system for wind power plants with aerogenerators provided with DC modular converters.
As schematically shown in FIG. 1, it is known that the wind plants use a plurality of aerogenerators 1, each one equipped with a respective wind impeller, 1a, which is supplied with the wind kinetic energy and can be mechanically controlled in such a way as to:                vary its alignment with respect to the wind direction;        vary the incidence of the blades which form impeller 1a, in such a way as to adjust the wind power converted into mechanical power.        
Inside the wind power plant, each aerogenerator is parallel connected to other aerogenerators 1 through a medium-voltage distribution electric network comprising a plurality of bipolar-type and DC electric lines L. The electric power produced by aerogenerators 1 and conveyed through the DC and medium-voltage distribution network, is transmitted through one or more medium or high voltage DC electric lines. Downstream with respect to these electric lines, there is a conversion station 3, which connects the wind power plant to the national transmission electric network.
Inside conversion station 3 there are at least an inverter 3a having an input supplied by DC line L and a respective output supplying the inlet of one or more step-up transformers 3b connected between the output of inverter 3a and the electric network.
Inside aerogenerator 1 there are, in order to create the drive train or power conversion chain (technically known as drivetrain), an electric generator 1b having a plurality of stator independent three-phase electric circuits (or three-phase voltage stars) 2 each of which is connected, through a respective three-phase line 2a, to a static conversion stage of electric power 4 of modular type and multi-leveled, from alternating voltage (present on stars 2) to direct voltage. Between electric generator lb and wind impeller la it is not interposed any step-up gear (gear box), in such a way as to reduce as much as possible the weight of the nacelle of aerogenerator 1 and to increase the energetic efficiency and the reliability of the whole system.
More in detail, as shown in FIG. 2, the stage of conversion 4 from alternating current (voltage) (AC) to direct current (voltage) (DC), comprises in its inner a plurality of AC-DC conversion modules 4a′-4a″″ each of which has a respective input connected to a respective three-phase line 2a and a respective pair of output terminals 4b 4c between which it is connected a respective condenser bank. The AC-DC conversion modules 4a′-4a″″ are of impressed voltage type formed by forced switching activated (IGB, IGCT or MCS) electronic devices and by respective recirculation diodes anti-parallel connected to them, in such a way as to create a three-phase bridge.
Each AC-DC conversion module 4a′-4a″″ is connected in series to the remaining AC-DC conversion modules, such that each of terminals 4b of each AC-DC conversion module 4a is directly connected to terminal 4c of the adjacent converter.
Each of the two AC-DC conversion modules, 4a′; 4a″″, at the edge of the series present a respective terminal 4b and respectively 4c connected at the output to conversion stage 4, in such a way as to form the medium voltage and DC bipolar line L.
The structure proposed for the control system for the wind power plant shown in FIG. 1, equipped with aerogenerators with drivetrain as in FIG. 2, bases itself on the control system used nowadays in the direct-driven aerogenerators with AC/DC/AC static converter of full scale type (i.e. at full power) shown in FIG. 3, which represents the state of the art for the last generations of wind turbines. In this, control structure a PLC 6 (programmable logic controller) operates as general controller of the wind turbine (Wind Turbine Controller) receiving as input a plurality of signals related to states, alarms and measures coming from the various sub-systems (not shown) that are integrated in the aerogenerator. PLC 6 manages, through its own output signals, respectively blade angle setting α, yawing angle δ of wind impeller la and rotation speed ω of generator 1b, from which depend the torque and therefore the power converted to the shaft by the generator. Furthermore PLC 6 provides the power and torque references to:                a first control stage 5 of the converter, which controls generator 1b through conversion stage 4; and        a second control stage 8 of the converter, installed in conversion station 3, which drives inverter 3a interfaced with the electric network. As shown in detail in FIG. 3, each converting stage 4 presents a respective first control stage 5, called “master” (i.e. principal) receiving as input a first signal VDC, related to the voltage present on the line L and a second signal CRIF of mechanical torque generated by electric generator 1b; these signals come from PLC controller 6. The purpose of the first control stage 5 is to adjust the impulses of the gate terminal of the IGBT or IGCT transistors provided inside AC-DC conversion modules 4a′-4a″″.         
PLC controller 6 also sends a further control signal to a second control stage 8, which drives the operation of inverter 3a. 
Inverter 3a has, as a matter of fact, an own control stage 8, capable of monitoring and keeping constant the above said voltage present on line L.
The control structure described in FIG. 3, used nowadays for each of aerogenerators 1, is present on each aerogenerator of the wind power plant.
Supposing that, as illustrated in FIG. 1, each AC-DC conversion module 4a produces on its output terminals 4b, 4c a direct voltage equal to 6 kV, and that AC-DC conversion modules 4a′-4a″″ placed in series are exactly four as shown in FIG. 2, it is clear that on line L is provided a direct voltage of 24 kV, thus in medium voltage, directed toward conversion station 3.
The group of aerogenerators 1, conversion station 3, electric lines L, PCL 6 and of the first, second conversion control stage 5, 8, form the so called MVDC system (medium voltage and direct current).
However, if a wind power plant is managed with these control systems, without bringing any modifications, there would be some drawbacks. In detail, if the voltage on line L is controlled and kept constant by the second control stage 8 of inverter 3a, it is not possible to precisely verify how it is distributed on each single AC-DC conversion module 4a. For example, supposing as before that each AC-DC conversion module 4a′-4a″″ has to nominally produced 6 kV DC, in such a way as to obtain on line L a 24 kV direct voltage or, owing to imbalances or lack of balance among the stars or among the conversion modules, the voltage on outputs 4b, 4c of each of the AC-DC conversion modules 4a′-4a″″ can assume values even very different (in a purely exemplificative way with the four modules that produce respectively 6 kV, 4 kV, 8 kV e 6 kV) without the total sum of the voltage produced by them having to change.
Since stars 2 to which the AC-DC conversion modules 4a′-4a″″ are connected are not perfectly identical as for the characteristics, the voltage lack of balance between a converter and the other is effectively provided with a certain frequency and, if provided in elevated values, can cause the breaking of AC-DC conversion module 4a′-4a″″ (for example of its IGBT or IGCT) owing to a too elevated voltage.
The use of a medium voltage and DC distribution network for the interconnection of aerogenerators, introduces another difficulty related to the protection system of the network, due in particular to the lacking of DC switches, adapted to beari voltage and power in such a way as to be used in a DC multi-terminal network as the one of FIG. 1.
As a matter of fact, in common DC electric networks, in case of failure, it is possible to carry out a quick sectioning of the section concerned by the failure without having the need of using particular constructive solutions in the AC switches positioned as protection for the electric lines; this is due to the fact that the alternating current, for each period, presents two instants wherein it has null value (supposing a sinusoidal current signal, these instants occur for an angle corresponding to 0 or 180°), in which the instantaneous value of current (zero, exactly) thus makes easier the opening of the switch and thus the sectioning of the circuit.
Vice versa, using a DC multi-terminal electric network, the lacking of passages through the zero of the medium voltage here present and thus of the current which passes on lines L, does not enable an efficient sectioning in case of failure, because the DC technology switches nowadays present on the market, would not succeed in effectively extinguishing the electric arc that would result in case of opening of the device in failure condition; therefore, they cannot be used.
In case of short-circuit then, at the moment the possible solution is to use the three-phase switches on the AC side of converters 4a and to coordinate their use with an opportune orientation of the blades of impeller 1a of the aerogenerators, in such a way as to slow down the wind impeller up to the total stop of the turbine. However, this shut-down procedure in case of failure on the DC network, has the disadvantage of being particularly long and thus of not enabling for a “shut-down” of the power plant in a reasonable time to avoid other aggravations of the failure already experienced and to keep the integrity of the modular converter and of the multi-pole generator safe.